Good darkroom practice necessitates that photographic solutions (developer, stop bath, fixing bath and wash) be maintained at the manufacturer's recommended temperatures. It is recommended that all solutions be kept at the same temperature including the water for the stop bath and wash. Differences in temperature can cause the film emulsion to wrinkle and break-up. Also, if a developer solution is too cool, then one or more of the chemicals may become partially inactive or slow acting. If the developer is too warm, then one or more of the chemicals will become too active. The established method of bringing the developer solutions to the correct temperature is to place them in a water bath wherein the water is at the correct temperature. The temperature of the water can be adjusted by mixing hot and cold tap water with the use of a thermometer. More sophisticated water baths are available which have a heating element immersed therein with an electronic temperature control.
It is common practice today to develop photographic film in a daylight processing drum into which the exposed photographic film is loaded prior to processing. The drums are designed so that solutions may be poured into and out of them without light entering the interior of the drum. The developing chemicals are poured into the drum and the drum is rotated to bathe the film in the developing chemicals. During the time of processing which may be only several minutes, the temperature of the solutions in the drum tends to change. It is desirable to be able to maintain the temperature of the drum and its contents substantially uniform by rotating the drum through a constant temperature bath.